a) Field of the invention
This invention relates to a gradient index optical element.
b) Description of the prior art
The gradient index optical element has aroused considerable attention as the optical element indispensable for the optical system of the coming generation due to its excellent ability to correct aberrations.
At present, various gradient index optical elements are available, not to speak of SELFOC (registered trademark) lenses and slab lenses already commercially available, which have been studied and developed by many enterprises and research institutions.
The gradient index optical element is adapted to provide its medium with index distribution, thereby rendering the medium per se in possession of a power (refracting power). The power depends on the index distribution and for the increase of the power, it is only necessary to increase the gradient difference (which is hereinafter referred to as .DELTA.n) of a refractive index n. Hence, increasing .DELTA.n is a subject given great study and development of the gradient index optical element and the study for increasing .DELTA.n is made by many researchers. For instance, in the optical element commercially available under the name of the SELFOC lens, .DELTA.n is increased by providing the concentration gradient of Tl through ion exchange.
Moreover, a lens configured as .DELTA.n.apprxeq.0.04 in which the concentration gradients of Pb and K are given by a sol-gel method (J. Non-Cry. Sol. 100, pp. 506-510, 1988) and a lens of .DELTA.n=0.03 derived from the concentration gradient of Ti or Ge [Elect. Lett. 22, pp. 99-100 (1986); Elect. Lett. 22, pp. 1108-1110 (1986)] are available.
Most of the up-to-date developments of the gradient index optical element rely on the approach that .DELTA.n and the outer diameter of the lens are made larger and are backward in provision of diminishing chromatic aberration of the optical element. Furthermore, in the gradient index optical element, although it is possible to decrease extremely the number of lenses when the optical element is conducted into an optical system due to its excellent correction ability for aberrations, discrepancy is encountered that the correction for chromatic aberration becomes difficult as the number of lenses decreases. Accordingly, to construct a lens system including the gradient index optical element and completely corrected for the chromatic aberration, there is the necessity of taking such a measure that an achromatic lens is incorporated in the lens system when occasion demands, and as such the merit of the gradient index optical system will be reduced by half.
Thus, in order to secure the lens system corrected for the chromatic aberration with a small number of lenses, it is of significance that the chromatic aberration per se produced for each lens is diminished. For this purpose, as requirements for the medium of the gradient index optical element, the following properties are desirable.
In a radial gradient index optical element, the refractive index of the medium varies according to a position (a distance from an optical axis) which a ray of light traverses so that the angle of refraction of the light ray changes. Assuming now that the Abbe's number ##EQU1## of the medium is uniform, the light ray is largely refracted in the region of high refractive indices as shown in FIG. 1A and therefore the extension caused by the difference of wavelength of the ray increases compared with that of the region of low refractive indices. In brief, if the Abbe's number .nu..sub.d is constant, the chromatic aberration (n.sub.F -n.sub.C) will increase as the refractive index n.sub.d increases. As such, for the reduction of the chromatic aberration (n.sub.F -n.sub.C), it is desirable that the Abbe's number is larger in the region of high refractive indices as shown in FIG. 1B. It follows from this that the behavior of changing from a high refractive index with low dispersion to a low refractive index with high dispersion is favorable for the property of the medium.
Further, an axial gradient index optical element, which has different refractive indices according to locations of the axial direction of the optical element, also has the same property as those of ordinary cemented achromatic lenses (doublets) illustrated in FIGS. 2A and 2B. That is, instead of the doublet having, with a boundary at a cemented surface, the high refractive index on one side and the low refractive index on the other side, the optical element is constructed so that the refractive index changes progressively in going from one surface to the other of the element, and thereby the optical element changing from the low refractive index with high dispersion to the high refractive index with low dispersion in going from an incident surface to an emergent surface as shown in FIG. 2C corresponds to FIG. 2A, whereas that changing from the high refractive index with low dispersion to the low refractive index with high dispersion in going from the incident surface to the emergent surface as in FIG. 2D to FIG. 2B, so that each of them has a similar property. Accordingly, in the axial gradient index optical element, it is said that, like the radial gradient index optical element, the property of changing from the high refractive index with low dispersion to the low refractive index with high dispersion is desirable.
This demonstrates that with respect to the n.sub.d -.nu..sub.d diagram shown in FIG. 3, the element whose optical characteristic changes in an A direction is superior in correction for chromatic aberration to that changing in a B direction (refer to Japanese Patent Preliminary Publication No. Sho 60-218614).
The gradient index optical elements developed at present, however, are aimed in most cases at the increase of .DELTA.n and in regard to those derived from the ion exchange such as Tl.sup.+ .rarw..fwdarw.K.sup.+ and Ag.sup.+ .rarw..fwdarw.Na.sup.+, the Abbe's number decreases as the refractive index (n.sub.d) becomes high. That is, this indicates the distribution in the B direction shown in FIG. 3.
Further, the elements constructed by the sol-gel method and the concentration gradient of Pb and Ti, although large in .DELTA.n, are such that the Abbe's number diminishes with increase of the refractive index (n.sub.d). That is, this demonstrates the distribution in the B direction of FIG. 3.
As a consequence of the foregoing, although such elements are large in .DELTA.n and have high correction abilities for spherical aberration, curvature of field, and distortion, they are not necessarily regarded as fine optical elements in view of the correction for chromatic aberration.
As for the distribution in the A direction, on the other hand, the elements constructed by means of the ion exchange of Li.sup.+ are available (Japanese Patent Publication No. Sho 59-41934). In such elements, however, the contribution to the refractive index of glass of Li.sup.+ per mol is not substantially made and as such where .DELTA.n is intended for increase, the concentration of Li.sup.+ must be made higher, with the result that it has been not achieved that .DELTA.n is increased without lowering the durability of glass.